Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels.
Classes of fuel cells include solid oxide fuel cells and solid oxide reversible fuel cells. Solid oxide reversible fuel cells allow reversed operation, such that water or other oxidized fuel can be reduced to unoxidized fuel using electrical energy as an input.
A solid oxide fuel cell (SOFC) system is a high temperature fuel cell system where an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The fuel cell typically operates at a temperature between 750° C. and 950° C. and enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream. The oxygen ions combine with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.
Fuel cell stacks may be either internally or externally manifolded for fuel and air. In an internally manifolded stack, the fuel and air is distributed to each cell using risers contained within the stack. Gas flows through openings or holes in the supporting layer of each fuel cell, such as the electrolyte layer, and gas separator of each cell. In an externally manifolded stack, the stack is open on the fuel and air inlet and outlet sides, and the fuel and air are introduced and collected independently of the stack hardware. For example, the inlet and outlet fuel and air flow in separate channels between the stack and the manifold housing in which the stack is located.
Fuel cell stacks are frequently built from a multiplicity of cells in the form of planar elements, tubes, or other geometries. Both fuel and air have to be provided to the electrochemically active surface, which can be a large surface. A fuel cell stack contains a gas flow separator plate that separates the individual cells in the stack. The gas flow separator plate separates fuel, such as hydrogen or a hydrocarbon fuel, flowing to the anode of one cell in the stack, from oxidant, such as air, flowing to the cathode of an adjacent cell in the stack. Frequently, the gas flow separator plate is also used as an interconnect made of or containing an electrically conductive material which electrically connects the fuel electrode of one cell to the air electrode of the adjacent cell.
It is difficult to achieve a reliable high fuel utilization in tall fuel cell stacks. Achieving high cell performance and maintaining that performance level for multiple years is desired for reaching economic viability in a commercial base load application.